In the design of semiconductor devices, it has been found advantageous in certain circumstances to provide a single crystal silicon layer of a predetermined thickness on a layer of an insulating material. Frequently, the layer of insulating material is itself formed on a silicon base layer or structure.
One method which has been proposed for forming such silicon on insulator devices comprises implanting oxygen into the surface of a silicon structure, and then subjecting the structure to an annealing step, thus forming a relatively thin layer of silicon on an insulating oxide layer which itself is on the silicon structure. Devices formed by this method are termed implanted buried insulator silicon on insulator structures. The silicon film formed on the oxide layer typically ranges in thickness from 800 to 2500 angstroms thick. These silicon film thicknesses, however, are insufficient to form many of the semiconductor devices desired to be formed on the insulating oxide layer.
Accordingly, it has been proposed to form a thicker single crystal or epitaxial silicon layer on the silicon film. However, application of conventional methods of forming an epitaxial silicon layer on an existing single crystalline silicon surface have proven unsatisfactory in the case of the silicon films of silicon on insulator structures. The conventional methods of forming an epitaxial silicon layer on an existing single crystalline silicon surface typically comprise a two-step process. The first step is a pre-treatment to remove residual silicon dioxide and/or a surface damage layer from the single crystalline silicon surface. The second step is a high temperature epitaxial silicon deposition step.
Several reasons lead to the unsuitability of these conventional epitaxial deposition processes to the silicon on insulator structure.
One reason is that the conventional pretreatment comprises a high temperature (i.e., at approximately 900.degree. C. to 1200.degree. C.) gas treatment with gasses comprising HCl or H.sub.2. It has been found that these pre-treatments can damage the surface of the silicon film, creating surface pits or, in other cases, entirely removing portions of the silicon film. In such cases, attempts to form an epitaxial silicon layer on the silicon film will result in an unsatisfactory silicon layer having pits or other irregularities. In fact, it has been found that epitaxial formation of a silicon layer will not occur at all in those regions where the silicon film has been entirely removed by the pretreatment.
It has also been found that the epitaxial deposition process itself, apparently by the combination of the high temperatures involved and the presence of H.sub.2 or HCl gases in the process, as well as other factors, tends to damage the silicon film and result in a formation of an unsatisfactorily irregular silicon layer on the silicon film.
Thus, it is desired to provide a method of forming an epitaxial silicon layer of a uniform or relatively uniform nature on the silicon film of a silicon on insulator device.